During monitoring of ECG signals, surgeons often use electrocautery devices. Electrocautery is a surgical technique which involves introducing high frequency current to a specific area of the body in order to remove unwanted tissue, seal off blood vessels, or to create a surgical incision. The instrument used to perform electrocautery is also known as an electrocautery. An electrocautery uses a high frequency, usually upwards of 100 kHz, to ensure that the patient's nerves and muscles are not stimulated. Lower frequencies could cause twitching and cramps, which would be a serious problem. Depending on the voltage used, the electrocautery can have varying effects on the patient's body. Thus, a very high level of radio frequency energy is coupled to the patient and in consequence to the inputs of the monitor.
Many implementations for reducing the amount of RF energy have been suggested, e.g. in U.S. Pat. No. 4,038,990, where coupled inductances are used together with capacitors, or in U.S. Pat. No. 5,217,010, where various LC filter alternatives are presented.
However, these known implementations do not consider the issue of RF currents which are carried by a separate shield. Especially, in ECG applications with shielded lead sets which are exposed to electrocautery RF energy, the problem arises that a current path is provided through the cable capacitance (inner conductor to shield). Thus, when all shields are connected together, a high current may flow through the series connection of these cable capacities, thus causing skin burnings at the electrode sites at the patient.
To reduce the risk for these burnings, it has been suggested reducing the current flowing into the lead wires by applying series protection elements in form of impedances (R and/or L) directly into the grabber of the lead set. However, a big disadvantage is that for a monitoring of respiration signals, which imply a low-impedance path to the patient, the ECG lead sets with built-in protection elements have to be replaced by such without protection elements. This is time-consuming and circumstantial, so it is highly desirable to avoid this procedure.
In U.S. Pat. No. 6,246,902 protection elements are no longer provided in the lead cable and the shields of the lead cable set are separated. Putting the protection elements in a separate block allows the usage of uniform unshielded lead sets for both operating and intensive care units. The shield conductors can also be fed through the block in a separated manner. Furthermore, in the shield conductors an inductance may be inserted to prevent current from flowing through the series connection of cable capacitances and in addition obtaining RF blocking in the shield while maintaining a good conductivity for low frequencies which obviously was the goal of this implementation.
However, the method of using inductors for separating and filtering shields has the disadvantages that the inductors are big and expensive and behave like capacitances above their resonance frequency. Thus, at higher frequencies they are again a good conductor. The fact that electrocautery frequencies are in the range of several hundred kHz up to 4 MHz makes it difficult to find an adequate inductor with a sufficient inductance to suppress the RF signals while its resonance frequency is still low enough. Moreover, together with (parasitic) capacitances, resonances can occur which degrade the RF suppression capability. Especially when there is a series resonance of the cable capacitance with the inductor, the currents can increase to an amount where burnings may occur again. Hence, especially when using lead cables of different length—or even unshielded lead cables—it is very difficult to assure that no resonance will occur at any condition.
In US 2009/149920 A1 a method is provided of using a resistive shield covering the lead of a pacemaker in order to avoid RF energy pickup during MRI procedures. The implementation of a costly and complicated resistive shield is only suitable if energy pickup is to be avoided over the whole length of the lead.